Why are the telephone booths red?

At one time the Blue Telephone Booth was a common site and was used by the Police (UK). This was well before the Mobile era. Dr Who's TARDIS (Time and Relative Dimension In Space) captures the acoustic telephone booth.

The Blue Telephone Booth was a staple in a number of early 1940-1950s films. Dr Who and his TARDIS keeps the memory of the Blue Booth alive. The classic wheezing sound of the TARDIS as it goes on another adventure.